A vigil for the 96 victims of the Hillsborough disaster.
Photograph: Christopher Furlong/Getty Images

Unearthing the truth about what happened at Hillsborough in 1989 would have been impossible without the Human Rights Act, according to one of the relatives who successfully fought against the original inquest ruling on the deaths of the 96 victims.

Under the Human Rights Act, a jury can be convened to assess the wider circumstances surrounding a death – something that had not been legislated for before it came into law.

Shah said: “Without the Human Rights Act we would never, ever have had the second inquest. Without the Human Rights Act we would never, ever have got the verdicts of unlawfully killed against all the parties that were culpable. It was an absolutely imperative piece of legislation.”

Her remarks are at the centre of an Amnesty International campaign warning that if the Human Rights Act is repealed when the UK leaves the EU, there could be miscarriages of justice.

Replacing the act with a UK bill of rights was part of the 2015 Conservative manifesto, but there have been reports that the much-delayed plan could be ditched. In August the justice secretary, Liz Truss, said it would still happen but gave no timeframe for it.

The campaign to save the act has also been backed by the widower of a woman killed by an IRA bomb, and a couple who used the legislation to uncover negligence that had led to their son’s death.

Other people involved include Gary McKinnon, who successfully fought extradition to the US on hacking charges, and a woman who used the act to uncover police errors committed before her mother was murdered.

Shah, whose brother survived the Hillsborough crush, said she was “extremely worried” about what could happen if the act was repealed. She said: “My fear is that if the Human Rights Act were to be abolished and replaced with the British bill of rights, which would be very limited, miscarriages of justice would go back to being far more easily perpetrated by the state.”

Alan McBride, whose wife, Sharon, died in the 1993 Shankill bombing, and who has since worked on community reconciliation, said the act was “extremely important” for the Northern Ireland peace process.

“What the Human Rights Act does is to give a safeguard. If you think you’re not getting anywhere through the normal course of law and order, at least you have this fallback,” he said.

“It’s enshrined in it that if your loved one is murdered you’re entitled to an investigation and be told about where that has got to. Without that, I don’t know where your redress would be.”

McKinnon, whose extradition was eventually blocked partly because he has Asperger’s syndrome, said the act played a key role in his case.

He said: “The Human Rights Act is important because it legislates for the protection of basic freedoms. These freedoms are unchanging, so the act itself should only be added to, not lessened or restricted or done away with.

“The act also gets a lot of bad press, with extreme examples of its use in terror cases, but we shouldn’t let a few bad interpretations get in the way of a necessary act.”

“It’s fundamental to everyone, in all walks of life,” Frank Robinson said of the act. “It opens access to justice for the man in the street. From the onset, all we wanted to know was the truth, and we were denied that by an inadequate first inquest. Using the Human Rights Act helped us achieve our goal.”

Asked about plans for a bill of rights, a Ministry of Justice spokeswoman said: “We will set out our proposals for a bill of rights in due course. We will consult fully on our proposals.”

As well as a series of videos highlighting the variety of people helped by the act, Amnesty commissioned a poll showing around two-thirds of people did not realise it had played a role in the new Hillsborough inquest or in the Good Friday peace process in Northern Ireland.

Kate Allen, UK director for Amnesty International, said the findings were understandable given the “misreporting and myth-making” about the act.

She said: “It’s no surprise that after they’ve been force-fed a diet of distortion, relatively few people know about the role the Human Rights Act has played in helping ordinary people up and down the country win historic legal battles.”

This article was amended on 15 November 2016 to correct the spelling of Shankill.